In mobile communication, communication is carried out using a downlink (DL) from a radio communication base station apparatus (hereinafter abbreviated as “base station”) to a radio communication mobile station apparatus (hereinafter abbreviated as “mobile station”) and an uplink (UL) from the mobile station to the base station. Downlink data and a downlink control signal are transmitted on a downlink and uplink data and an uplink control signal are transmitted on an uplink.
The uplink and the downlink are associated with each other, and, when, for example, ARQ (Automatic Repeat Request) is applied to downlink data, the mobile station feeds back a response signal indicating an error detection result of the downlink data to the base station using the uplink. The mobile station carries out a CRC (Cyclic Redundancy Check) check on downlink data and feeds back ACK (Acknowledgment) when CRC=OK (no error) or NACK (Negative Acknowledgment) when CR=NG (with error), to the base station, as a response signal. This response signal is transmitted to the base station using an uplink control channel such as a PUCCH (Physical Uplink Control Channel).
Furthermore, the base station transmits control information for reporting a resource allocation result of the downlink data to the mobile station. The control information is transmitted to the mobile station using a downlink control channel such as a PDCCH (Physical Downlink Control Channel). Each PDCCH occupies one or a plurality of CCEs. When one PDCCH occupies a plurality of CCEs (Control Channel Elements), one PDCCH occupies a series of a plurality of CCEs. The base station allocates one of a plurality of PDCCHs to each mobile station according to the number of CCEs necessary to report control information, maps control information to physical resources corresponding to the CCEs (Control Channel Elements) occupied by each PDCCH, and transmits the physical resources.
Furthermore, a study is underway to associate CCEs with PUCCHs for efficient use of downlink communication resources. Each mobile station can determine a PUCCH to use to transmit a response signal from that mobile station, from the CCE corresponding to the physical resource to which control information directed to that mobile station is mapped.
Since the uplink and downlink are associated with each other in this way, when a plurality of communication systems are preferred to be mixed, allocating an uplink and a downlink to each communication system results in a problem of causing a shortage of frequency resources. Furthermore, when a new communication system is added in a band in which an old communication system is operated, the new communication system is preferably used as is without making any change to the mobile station of the old communication system. Patent Literature 1 proposes a frequency overlay system as a method to solve these problems.
According to Patent Literature 1, when an old communication system and a new communication system are mixed together for the purpose of improving frequency utilization efficiency, the new communication system is designed to cover the frequency of the old communication system, and the new communication system performs scheduling by including the frequency of the old communication system. Furthermore, the accuracy of channel estimation is improved by lowering the correlation between a preamble channel (reference signal) used in the old communication system and a preamble channel (reference signal) used in the new communication system. Furthermore, control channels are provided separately for the new communication system and the old communication system so as to transmit signals in different frequency bands. According to the method disclosed in Patent Literature 1, the uplink and the downlink both operate in the same frequency arrangement in the old communication system and the new communication system.
Furthermore, Non-Patent Literature 1 proposes a frequency arrangement in which an LTE system and an LTE+ system coexist, assuming the old communication system is “LTE” and the new communication system is “LTE+.” According to Non-Patent Literature 1, both the uplink and the downlink of the LTE system are arranged in low frequencies. However, in such an arrangement, the center frequency of the downlink of the LTE system is different from the center frequency of the downlink of the LTE+ system. Therefore, for initial synchronization or HO (handover) control, it is necessary to transmit control channels such as an SCH (Synchronous Channel) and BCH (Broadcast Channel) in separate frequencies in both LTE and LTE+.
As a method for solving this problem, there is a method of sharing a center frequency on a downlink, arranging a downlink for LTE in the center and allocating a DL band for LTE+ and a DL band to overlap one another. FIG. 1 shows an arrangement example of a downlink band in this case. FIG. 1 is an example where a DL band for LTE is 10 MHz and a DL band for LTE+ is 40 MHz. As shown in FIG. 1, the DL band for LTE+ is arranged extending to both sides centered on the center frequency of the DL band for LTE. Furthermore, an SCH and BCH of LTE are transmitted using the center frequency of LTE. The SCH is shared between LTE and LTE+. Furthermore, using the differential of the BCH or the like as a BCH+, the BCH+ is transmitted in the DL band for LTE+.
FIG. 1 shows an arrangement of a UL band corresponding to a DL band. As shown in FIG. 1, the center 10 MHz of a UL band is allocated for a UL band for LTE, and 40 MHz, overlapping with the UL band for LTE, is allocated for a UL band for LTE+. Furthermore, a PUCCH and PUCCH+ are arranged at both ends of the uplink of each system. The PUCCH is an uplink control channel for LTE, and the PUCCH+ is an uplink control channel for LTE+. In the LTE system, PUCCHs are arranged within 5 MHz on the left side and right side of the center frequency, while in the LTE+ system, PUCCH+'s are arranged within 20 MHz on the left side and right side of the center frequency.